One of the most poorly understood essential components of the immune system function is the process through which an organism maintains tolerance to self gene products and responsiveness to pathogens. A particularly promising approach to understanding how this delicate balance is achieved is study of polymorphic non H-2 genes (minor histocompatibility (H) genes) that can be detected because their products are antigenic to genetically different individuals. Using this approach we will address two critical questions: (1) How many of these types of genes are there and (2) what is their molecular basis? determination of the numbers of these genes will be approached by carrying out a breeding and experimental scheme that will provide an estimate of the numbers of polymorphic genes whose products can stimulate class I MHC- restricted (cytotoxic) T cells (Tc) and class II MHC-restricted (helper) T cells (Th) to answer the following significant questions: Are there many or few minor H genes whose products stimulate Th and Tc? Is there overlap or random distribution of minor H genes defined by Th vs. Tc activity? Are numbers of minor H genes significantly influenced by evolutionary distance or Mls- disparity? The molecular basis of these types of genes will be approached in two ways. First, gene transfection experiments will address whether the well- defined gene beta2-microglobulin (B2m) is a minor H gene whose products are recognized as antigen by minor H antigen-specific Tc and will determine the molecular basis of the B2m antigen. Second, Tc will be used as genetic probes in gene transfection experiments involving screening cDNA and cosmid libraries and isolating the minor H gene cdH-4 whose products are recognized by Tc. Ensuing molecular analysis will determine both the structure of the cdH-4 gene and the molecular basis of cdH-4 antigenicity. Included in these studies will be determination of whether the antigens are produces through normal transcriptional events or by short autonomous transcriptional units (peptons). The information obtained should clarify many important issues concerning minor H genes and will also provide a refined approach for cloning other genes whose products elicit T cell activity.